As best I can figure out, the IF NOT mess refers to situations other than shooting involving BUA/camps ??? Which is to say, the MOST COMMON situation. Naturally, you have formatted the most important-to-understand rule into an incomprehensible mess.

In fact, I can't make any sense of what the first "if not" refers to. The previous refers to Measurement, and the If Not mess refers to Priority.

HOW ABOUT THIS REPHRASING:

DISTANT SHOOTING
Range is up to 500 paces (5 BW) for Artillery and up to 200 paces (2 BW) for Bows and War-Wagons.

Measurement is 1) between the closest points of the shooting edge (any edge of a War Wagon or the front edge of Bows or Artillery) and of any 1 enemy element edge (the target edge) that is within 50 paces (1⁄2 BW) of directly in front,

except that 2) shooting at or from a BUA or camp is between its nearest point and the shooter or target.

Target priority for Bow and WWg are targets which are 1) in their DZ, 2) shooting at them, 3) any eligible target. Artillery always chooses its target and can shoot through enemy Psiloi.

VS DRAFT PHRASING:

DISTANT SHOOTING
Distant shooting is limited to Artillery up to 500 paces (5 BW) and Bows and War-Wagons up to 200 paces (2 BW), measured between the closest points of the shooting edge (any edge of a War Wagon or the front edge of Bows or Artillery) and of any 1 enemy element edge (the target edge) that is within 50 paces (1⁄2 BW) of directly in front, except that shooting at or from a BUA or camp is between its nearest point and the shooter or target. If not, Bows and WWg must shoot at a target in their DZ or, if not, shooting at them, otherwise choose any eligible target. Artillery always chooses its target and can shoot through enemy Psiloi.